This invention relates to data communication systems and, more particularly, to digital frequency-shift-keyed modulators for providing sinusoidal wave output signals of different frequency indicative of the occurrence of marks and spaces in a data signal.
The impact of computer proliferation and distributed intelligent systems has increased the amount and availability of information or data to be processed, stored, and transmitted. A convenient technique for transmitting data involves modulators for converting data signals into voice frequency tones for transmission over ordinary telephone lines. Numerous frequency-shift-keyed modulators of both the analog and digital variety have been devised by those skilled in the art as one means of implementing this technique.
Since the anticipated demand of such modulators will increase with time, it would be highly desirable to have designs compatible with the new large scale integration logic technologies in terms of fabrication and powering requirements. In addition, such a modulator should also be relatively inexpensive to manufacture as well as meeting the signaling requirements for suppression of out-of-band tones (e.g., harmonics). Furthermore, such a modulator should provide a smooth transition, called quasi-continuous, when shifting from a mark frequency to a space frequency tone and vice versa.
It is an object of the present invention to provide a relatively inexpensive and simple frequency-shift-keyed modulator and, in particular, for accomplishing these aims, to provide a frequency-shift-keyed modulator which is principally digital in construction and function.
A related object of the invention is to provide a frequency-shift-keyed modulator which produces a sinusoidal output signal by directly low-pass filtering a sequence of binary digits and therefore obviates the digital-to-analog converter employed in conventional frequency-shift-keyed modulators.
A further object of the invention is to provide a balanced loss-pass filtering arrangement for inherently and conveniently eliminating even ordered harmonic frequency components by cancellation or destructive interference.